Gallery bug fixes

[marjisound]

• image caption titles are missing
• frontend shouldn't add empty h2 to the image captions
• app lightbox caption link colour
• app main media picture cropped
• discussion should occure after merchandising high, and fix the border colour for discussion
• The sub meta colouring is different between AR & frontend (what colour is correct)
• in-body images are requested at width 620 as opposed to 1020 (their CSS width; frontend does 1010, close enough), this is a special gallery width, so probably DCR treats them as weighting (role in DCR?) inline?
• labs header visible on non-labs galleries:
  
• top of Main media covers whole header and launches lightbox when clicked (just couple of things push through z-index)
• [all immersives?] Main media caption should place icon on the same line as text (example):
  
• gremlin at the bottom:
  
• series badge at top of headline should be pillar-coloured
• [all immersives?] Main media caption font size discrepancies? (Main/body: DCR galleries 14/12, frontend galleries 13/12, immersives 14/14)
• [all immersives?] We seem to have lost “Main image: ” prepended to Main image caption on mobile breakpoints in all immersive scenarios? Those captions may now look like they describe who-knows-what. Maybe standardise to camera icon in this scenario (remember to remove it in empty caption+Display credit:OFF case)
• refactor GalleryLayout to abstract a lot of the details into sub-components. Look at this comment Jamie left